The present invention relates to a powder coating composition capable of providing a matte or low gloss finish on a variety of substrates. The gloss of a cured powder coating is typically described using terms including “matte”, “low gloss”, and “mid gloss” finishes. In general, gloss is expressed as a percentage of intensity of the reflected light with respect to the intensity of the incident light at a specified angle between the incident light beam and the planar test surface. The terms “matte”, “low gloss”, and “mid gloss” are defined herein according gloss measured according to ASTM D523 at an angle of 60°. Specifically, a “matte” finish has a 60° gloss less than 20 units; a “low gloss” finish has a 60° gloss of 20 to less than 40 units; and a “mid gloss” finish has a 60° gloss of 40 to less than 60 units.
Gloss reduction in powder coatings may be achieved by producing a fine surface morphology that scatters incident light, resulting in a lower percentage of reflected light. This low percentage of reflected light gives the appearance of matte or reduced gloss. A variety of methods have been developed to provide low gloss powder coatings, including the incorporation of fillers or extenders, the incorporation of incompatible ingredients such as waxes, dry blending of different formulations, and through the incorporation of matting agents.
The incorporation of fillers or extenders has been used to produce gloss in the mid-gloss range, however this technique does not readily provide for coatings to be produced with a smooth matte or low gloss finish. Also, the incorporation of fillers can lead to coatings with reduced physical properties including impact, flexibility, and adhesion due to reduced binder concentrations.
The incorporation of incompatible ingredients such as waxes is also commonly used to produce mid gloss finishes. As with the incorporation of extenders, this technique does not readily provide for coatings with matte or low gloss. Incorporation of waxes often leads to the formation of a surface film as waxes migrate over time, and removal of the surface film can expose a glossy surface underneath.
Dry blending of two powders that have different reactivities or are immiscible has been described in, for example, U.S. Pat. No. 3,842,035 to Klaren. Dry blending requires an additional manufacturing step and the resulting dry blended components can separate over time in a conventional powder coating application and recycle process. The separation of components during the application and recycle process can yield fluctuations in gloss and curing efficiency.
The incorporation of matting agents has also been used to provide a coating with matte or low gloss appearance. The underlying process has been described as one of competing reactions or varying rates of separate reactions. The use of reactive matting agents is described in, for example, European Patent Application Nos. 72,371 A1 to Holderegger et al., and 44,030 A1 to Gude et al.; European Patent Nos. 165,207 B1 and 366,608 B1 to Lauterbach; U.S. Pat. No. 5,684,067 to Muthiah et al., U.S. Pat. No. 5,786,419 to Meier-Westhues et al.; D. H. Howell in “The Technology, Formulation and Application of Powder Coatings”, J. D. Sanders, Ed., John Wiley and Sons in association with SITA Technology Limited: London, England 2000. Vol. 1, pages 152-178; C. Grob and C. Rickert (2000) Water-Borne, Higher-Solids, and Powder Coatings Symposium, New Orleans, 1-3 Mar. 2000, pp 337-349; P. A. Chetcuti, B. Dreher, and P. Gottis, Mod Paint Coatings (1995), volume 85, no. 7, pages 28-32; J. J. Salitros and R. Patarcity, Proc. Water-Borne, Higher-Solids, Powder Coat. Symp. (1992), 19th, pages 517-526; and J. Schmidhauser and J. Havard, Proc. Int. Waterborne, High-Solids, Powder Coat. Symp. (2001), 28th, 391-404.
Gloss has been lowered in epoxy powder coating systems by using reactive matting agents such as cyclic amidines and amidine salts as described, for example, in European Patent Application No. 44,030 to Gude et al., and in Ciba-Geigy Product Literature “Matting Agents/Hardeners for Powder Coatings”. This is commonly known as “veba” technology.
Another approach is the addition of acid anhydride containing materials, addition of polyacids, or by dry blending two powder coatings with different reactivities as described in U.S. Pat. No. 3,842,035 to Klaren. Howell, cited above, refers to the incorporation of reactive matting in powder coatings to achieve low gloss and states that gloss level of less than 20% are attainable but depend on the curing conditions, which have to be carefully controlled in order to ensure reproducibility.
Examples of acid functional reactive matting agents used in epoxy systems and polyester/epoxy hybrid systems include styrene maleic anhydride copolymers or esterified styrene maleic anhydride copolymers as described in the Salitros et al. and Schmidhauser et al. references cited above. The styrene-maleic anhydride copolymers are described as functioning as matting agents at cure temperatures of at least 400° F. in polyester/epoxy hybrid systems.
There remains a need for a coating composition that provides low gloss at low curing temperatures and consistently provides low gloss or matte gloss over a broad temperature range.